ABSTRACT

Offshore pipelines are typically protected from corrosion by a combination of factory applied high performance coatings, cathodic protection and field applied joint coating prior to final installation. Recently, economic incentives have been driving some parties to consider eliminating the offshore pipeline field joint coatings and to leave the welded joints bare and protected only by the cathodic protection system. Concerns are raised that this approach may significantly impact the integrity of the pipelines over the typical design lives of 25 to 40 years. Areas of concern include increased risk of microbially induced corrosion, risk of creation of crevices at edges of mill coating without field joint coating, increased risk of disbonded coatings, and increased risk of hydrogen embrittlement on parent pipe, HAZ and welds without field joint coatings. Industry has been aware of some of these risks for some time with numerous studies relating to hydrogen inducted cracking already published. Severe disbondment of mill coating at normal cathodic potentials and simulated bare field joint has been observed in 20 month ambient temperature testing.

ABSTRACT

All organic materials, including coatings, degrade due to complex chemical changes. As these materials degrade, so their properties will change. Pipeline external anti-corrosion coatings are no different as their physical properties change over time. The issue is that there isn’t a complete understanding of how the physical properties change. It is known that many factors or mechanisms can affect the properties of materials. These include temperature, UV, oxygen, stress, expansion/ contraction, exposure environment (air/water/soil), etc.

As examples:

The polypropylene (PP) topcoat formulation for pipe coating contains UV stabilizers and fillers to reduce the impact of open exposure to sunlight. Given its chemical composition and molecular structure, PP is naturally resistant to moisture but ages in oxygen (air), particularly in combination with UV induced free radicals. The degradation rate increases with temperature; the ageing process reduces the strength and flexibility of the polymer.

The fusion bonded epoxy (FBE) primer layer is mostly protected from the elements by the polypropylene layers but it is exposed at pipe ends. FBE will suffer from ‘chalking’ under UV and will take up moisture which over time reduces its adhesion to steel. The ends of the coating are particularly vulnerable to disbondment caused by weakening FBE adhesion (due to moisture absorption) together with the pulling effect of the PP that was cooled after extrusion on to the pipe (residual stress).

During storage however, the combination of these effects are expected to impact the integrity of the coating over time. Such negative effects cannot be stopped but their rate of progress can be reduced by using preservation methods.

Preservation methods address specific, critical areas of the coated pipeline, and they are targeted to reduce the rate of degradation. However, they were not designed to act against more complex mechanisms intrinsic to the natural ageing of organic coatings.

The Shawcor Approach To Preservation

Shawcor have noted the lack of a systematic approach to preservation:

The lack of standardization, recommended practices or a scientific approach to preservation

The difference between quality control and fitness-for-purpose testing – current standards are not designed to test aged coatings

Shawcor have taken their internal guidelines, plus lessons learned from previous projects, to establish a set of proposals based on data. Shawcor share the tools and procedures that form the present solutions, and discuss the benefit to the industry of following the process.

Preservation Strategies

To minimize, if not eliminate, the potential effects mentioned above, Shawcor proposes:

to use barriers limiting access of moisture to steel, FBE & IFC e.g. end seal tape

to use barriers limiting ingress of moisture and environmentally carried particles into the bore of a pipe, e.g. end caps

to use product limiting moisture presence in the bore of a pipe, e.g. desiccant

to use barriers limiting UV degradation of thermoplastics, e.g. end seal tape and tarpaulins

to recommend coating termination geometries that are better at minimizing end effects, or at least allow some sacrificial loss of coating without impairing performance

Future Strategies

There is a lack of knowledge in the industry on how individual properties will change over time when coated pipelines are exposed to external stresses due to changes in temperature, mechanical loading and environmental conditions.

During its participation at AMI Pipeline Coating, Shawcor will discuss these topics and propose recommended actions for the industry. Join us and find out more about our solutions!

ABOUT AMI PIPELINE COATING 2019

With a truly international participation covering the entire supply chain, Pipeline Coating 2019 offers unmatched opportunities to network, exchange experience and learn through interaction with colleagues, suppliers, customers and competitors.

AMI Pipeline Coating 2019

TOPIC HIGHLIGHT :

SHAWCOR PRESENTATION SCHEDULE:

February 13, 2019 at 14:20 - Impact of Exposed Field Joints on the Integrity of Offshore Pipelines

February 14, 2019 at 12:20 - Recommendations for the Preservation of Pipeline Coating

The Pipeline Coating conference has proved extremely valuable by attracting an audience focused on all aspects of steel pipeline coatings. Pipeline Coating 2019 presents an unrivaled opportunity to meet and network with experts and to learn more about the application of existing and developing protection technologies.